An internal combustion engine may include a catalytic converter to process engine exhaust gases into N2, H2O and CO2. Once the catalyst has reached a catalyst light off temperature, the catalyst provides a threshold exhaust gas conversion efficiency. The catalyst light off temperature is much high than ambient temperature. Consequently, the catalyst may not convert exhaust gases efficiently as is desired after an engine has started from ambient conditions. One way to increase catalyst efficiency during engine starting is to retard spark timing from a base spark timing shortly after engine starting so that a greater percentage of heat released from combustion of air and fuel in the cylinder reaches the catalyst. Further, an amount of air inducted to engine cylinders can be increased to further increase catalyst temperature sooner after engine start. However, increasing an amount of air inducted to the engine reduces vacuum produced by the engine. As a result, the engine may be able to supply less vacuum to vacuum consumers, such as vehicle brakes, during engine starting. If a driver applies vehicle brakes after engine starting when low vacuum is present, the driver may experience hard brake pedal feel. Thus, it may be desirable to provide a way of heating a catalyst while providing vacuum to vacuum consumers during low driver demand torque conditions.
The inventors herein have recognized the above-mentioned issues and have developed an engine operating method, comprising: combusting air and fuel within cylinders of an engine at an idle speed while providing engine torque insufficient to rotate the engine at the idle speed; and rotating the engine at the idle speed via torque produced by a motor while providing engine torque insufficient to rotate the engine at the idle speed in response to a request to heat a catalyst.
By commanding a motor to rotate an engine while the engine is combusting air and fuel at an idle speed and producing torque insufficient to rotate the engine, an increased amount of thermal energy may be provided to heat a catalyst. For example, spark timing of the engine may be retarded from minimum spark timing for best engine torque (MBT) so that the engine produces torque insufficient to rotate the engine at idle speed. The retarded spark timing delays ignition so that additional heat flows out of the engine to the catalyst. Further, the engine may be rotated via the motor to generate additional vacuum. By lowering the load on the engine via the motor, the engine may operate with less air so that the throttle may be closed further to generate additional vacuum for vacuum consumers on board the vehicle.
The present description may provide several advantages. Specifically, the approach may reduce catalyst heating time and reduce engine emissions. Further, the approach may provide increased vacuum when vacuum may be difficult to produce. Further still, the approach provides a way of heating a catalyst via the vehicle's battery without having to install an electrical heater in the vehicle's exhaust system.
For example, by motoring an engine with 2 kW of energy provided by a motor, the engine is able to produce approximately 2 kW of additional catalyst heat at the same intake manifold vacuum level if the engine provided heat to the catalyst without the motor. Or, the engine can produce a deeper vacuum at the same catalyst heating level. This approach provides a surprising serendipitous result. Specifically, since the engine is being motored primarily with an electric motor instead of combustion torques, it smoothness increased remarkably and allows even greater spark retard where without this feature, the engine would encounter a problematic combustion stability limit while spark was being retarded. Thus, rapid catalyst heating may be achieved while maintain a desired intake manifold vacuum while experiencing increased engine smoothness during catalyst heating. Further, the approach enables running the engine at very low Indicated Mean Effective Pressure (IMEP), which is known to produce low hydrocarbon emissions and is one objected for fast catalyst light-off. As such, the catalyst heating is performed while producing a fewer hydrocarbon emissions than would occur if the combustion pressures were spinning the engine.
The intake vacuum is provided by rotating the engine via the motor is useful for crankcase ventilation, fuel vapor purge, low pressure EGR, exhaust heat recovery, vacuum actuators (such as brake boost, adjustable engine mounts, 4×4 hub locks).
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.